* The `csrf.Protect` middleware/handler provides CSRF protection on routes
attached to a router or a sub-router.
* A `csrf.Token` function that provides the token to pass into your response,
whether that be a HTML form or a JSON response body.
* ... and a `csrf.TemplateField` helper that you can pass into your `html/template`
templates to replace a `{{ .csrfField }}` template tag with a hidden input
field.

gorilla/csrf is easy to use: add the middleware to individual handlers with
the below:

... and then collect the token with `csrf.Token(r)` before passing it to the
template, JSON body or HTTP header (you pick!). gorilla/csrf inspects the form body
(first) and HTTP headers (second) on subsequent POST/PUT/PATCH/DELETE/etc. requests
for the token.

Note that the authentication key passed to `csrf.Protect([]byte(key))` should be
32-bytes long and persist across application restarts. Generating a random key
won't allow you to authenticate existing cookies and will break your CSRF
validation.

Here's the common use-case: HTML forms you want to provide CSRF protection for,
in order to protect malicious POST requests being made:

Note that the CSRF middleware will (by necessity) consume the request body if the
token is passed via POST form values. If you need to consume this in your
handler, insert your own middleware earlier in the chain to capture the request
body.

You can also send the CSRF token in the response header. This approach is useful
if you're using a front-end JavaScript framework like Ember or Angular, or are
providing a JSON API:

In addition: getting CSRF protection right is important, so here's some background:

* This library generates unique-per-request (masked) tokens as a mitigation
against the [BREACH attack](http://breachattack.com/).
* The 'base' (unmasked) token is stored in the session, which means that
multiple browser tabs won't cause a user problems as their per-request token
is compared with the base token.
* Operates on a "whitelist only" approach where safe (non-mutating) HTTP methods
(GET, HEAD, OPTIONS, TRACE) are the *only* methods where token validation is not
enforced.
* The design is based on the battle-tested
[Django](https://docs.djangoproject.com/en/1.8/ref/csrf/) and [Ruby on
Rails](http://api.rubyonrails.org/classes/ActionController/RequestForgeryProtection.html)
approaches.
* Cookies are authenticated and based on the [securecookie](https://github.com/gorilla/securecookie)
library. They're also Secure (issued over HTTPS only) and are HttpOnly
by default, because sane defaults are important.
* Go's `crypto/rand` library is used to generate the 32 byte (256 bit) tokens
and the one-time-pad used for masking them.